Try stock xchange:
www.sxc.hu. A really good, extensive range of photographs, the majority without usage restrictions of any kind but quite a few with restrictions (require written permission etc.) You'll need to spend a bit of time searching round the site as the quality varies, but that's half the fun!
Depends what you're after of course, and how creative you are with image selection, manipulation, cropping and so on. Sometimes you just can't beat a resource like Getty Images, in which case you should get your client to agree a stock photography budget from the outset.
Hope that helps.
Cheers,
Simon